Uniformity adjustment method for a diode-laser line-projector

ABSTRACT

In a line projector a diode-laser beam having an elliptical cross-section is projected onto a Powell lens which spreads the beam to form a line of light. Distribution of power along the line of light is adjusted by rotating the diode-laser beam with respect to the Powell lens.

PRIORITY

This application is a divisional of U.S. patent application Ser. No.13/628,756, filed Sep. 27, 2012, the disclosure of which is incorporatedby reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to optical devices forprojecting a line of light from a diode-laser (laser line projectors).The invention relates in particular to laser line projectors wherein anacylindrical lens is used to spread light into a fan of rays forming thelength of the line of light.

DISCUSSION OF BACKGROUND ART

Laser line projectors find extensive use in machine vision applications.In these applications, the line of light is projected on a surface andlight reflected from the surface is received by a detector. The surfaceis scanned relative to the detector and the detector output iselectronically processed to build up a three-dimensional (3D) image ofthe surface. It is very important in such an application that lightuniformity in the length direction of the projected line be uniform. Themore uniform the illumination on the surface, the more faithful areproduction of the surface the 3D-image will be.

The most common laser line projectors used in machine visionapplications are based on a lens having an acylindrical surface(acylindrical lens) usually referred to by practitioners of the art as aPowell lens, after the inventor. Such a lens and an arrangement forusing the lens for projecting a line of light are described in detail inU.S. Pat. No. 4,826,299, the complete disclosure of which is herebyincorporated herein by reference. Later variations and applications ofthe Powell lens are described in U.S. Pat. Nos. 5,283,694; 5,629,808;7,167,322; and 7,400,457, among others.

A typical basic configuration of a laser line projector includes adiode-laser delivering a laser-beam characterized as having a fast-axisand a slow-axis perpendicular to each other. The diode-laser is followedby a positive lens, and then the acylindrical beam-shaping lens orPowell lens.

The Powell lens itself is characterized in having a first axis in whichacylindrical surface has optical power, and a second axis, perpendicularto the first axis, in which the acylindrical surface has zero opticalpower. The diode-laser is invariably arranged such that the fast- andslow-axes of the diode-laser are aligned precisely (at zero degrees)with the respectively first and second axes of the Powell lens, orvice-versa. The Powell lens spreads the laser beam in the first-axis ofthe lens such that the power in the beam is spread linearly as afunction of spread-angle (fan angle) to provide a uniform or “flat-top”illumination along the spread beam. The positive lens typicallyconfigured and positioned to create a focus in the second axis toprovide a uniform line of light at about the focus position, i.e.,within the focal depth. The positive lens can also be positioned to,collimate or diverge, the beam in the other axis.

In this line projection arrangement the Powell lens is designed for aparticular laser-beam size incident on the lens. If the beam incident onthe lens does not match this size, the lens will not provide optimumuniformity along the line of light. Indeed, the uniformity ofillumination in the line is sensitive even to relatively smallvariations in beam size incident on the lens.

By way of example FIG. 1 schematically illustrates calculated intensityas a function of fan-angle and beam size in a line spread by a Powelllens optimized for a beam size of 2 mm. The intensity for a beam of thenominal (optimum) beam size is depicted by a fine solid line. Theintensity distribution for a beam size 15% smaller than nominal isdepicted by a bold dashed line. The intensity distribution for a beamsize 15% larger than nominal is depicted by a bold solid line. Thisdistribution is usually termed a “bat-ear” distribution or a “pitchfork”distribution.

A manufacturer of laser line projectors is required to provide projectedlines of diverse wavelengths and powers to satisfy the demand ofdifferent users. In order to satisfy such diversity, a manufacturer mustemploy different diode-lasers possibly from different manufacturers in aline-projector product line. This will result in a fixed opticalarrangement of the type described above with a range of differentbeam-sizes at the Powell lens. Absent measures to deal with this, awide, unacceptable variation in intensity distribution would result.

Various solutions to the problem of varying beam characteristics ofdiode-lasers are employed. By way of example the beam size may bemanipulated by additional optical elements between the diode-laser andthe Powell lens, i.e., a zoom lens may be used as the positive lens.Diode-lasers of any one type may be sorted to find those havingbeam-divergence divergence within a design tolerance. In addition,Powell lenses are often reconfigured (re-polished), by trial and error,to match particular diode-laser beam characteristics. These solutions,however effective, can consume a large amount of time or be costlyimplement. There is a need for a simpler solution for accommodating awide range of diode-lasers in a particular diode-laser line projectordesign.

SUMMARY OF THE INVENTION

A diode laser line projector device is disclosed. In one preferredembodiment, the device includes an elongated housing with first andsecond rotatable sections. A diode laser is mounted within the firstsection of the housing. An acylindrical lens is mounted within thesecond section of the housing.

During manufacture, the acylindrical lens is rotated about thepropagation axis of the beam to vary the azimuthal angle of the lenswith respect to the fast and slow axes of the diode laser. Duringrotation, the uniformity of the intensity of the line of light projectedby the lens is measured. When the desired intensity pattern is achieved,the housing is locked to define a fixed azimuthal position between thelens and the axes of the diode laser.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain principles of the presentinvention.

FIG. 1 is a graph schematically illustrating intensity as a function offan-angle for a nominal beam-size for which the Powell lens is designedand for beams larger and smaller than the nominal size.

FIG. 2 is a three dimensional view schematically illustrating apreferred embodiment of diode-laser line projection apparatus withprovision for adjusting intensity distribution in the projected line inaccordance with the present invention, the apparatus including adiode-laser followed by a positive lens, followed by a Powell lens, thediode-laser being rotatable with respect to the Powell lens, forrotating the diode-laser beam with respect to the Powell lens.

FIG. 2A is a three dimensional view similar to the view of FIG. 2,schematically depicting the diode-laser beam rotated non-orthogonallywith respect to transverse optical axes of the Powell lens.

FIG. 3 schematically illustrates an exemplary diode-laser beamorthogonally aligned with axes of a Powell lens, with the exemplary beamhaving a height greater than the height for which the Powell lens isconfigured.

FIG. 4 schematically illustrates the exemplary beam of FIG. 3 rotated atan angle of 30° to the axes of the Powell lens such that the beam has aneffective height at the Powell lens equal to the beam height for whichthe Powell lens is configured.

FIG. 5 is a graph schematically illustrating effective beam-size as afunction of rotation angle of an elliptical beam with respect to Powelllens axes as depicted in FIG. 4.

FIG. 6 is a three-dimensional view schematically illustrating onepractical example of diode-laser line projection apparatus configured toaccommodate the beam uniformity adjustment method of the presentinvention.

FIG. 7 is a longitudinal cross-section of the example of FIG. 6.

FIG. 8A is a graph schematically illustrating measured relative power asa function of line length for a diode-laser beam having a fast-axis anda slow-axis projected from a 515 nm wavelength diode-laser onto a Powelllens for spreading the beam to form the line, with the slow-axis of thebeam aligned with the Powell lens.

FIG. 8B is a graph schematically illustrating measured relative power asa function of line length when the diode-laser beam of FIG. 8A has thefast-axis thereof aligned with the Powell lens.

FIG. 8C is a graph schematically illustrating measured relative power asa function of line length when the diode-laser beam of FIG. 8A has theslow-axis thereof aligned at angle of 30° to the Powell lens.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, wherein like components are designated bylike reference numerals, FIG. 2 schematically illustrates a preferredembodiment 20 of diode-laser line projection apparatus with provisionfor adjusting intensity distribution in the projected line in accordancewith the present invention. Apparatus 20 includes a diode-laser assembly22 in the “can” form typical of diode-lasers from commercial suppliersof diode-lasers. A diode-laser beam (here with only propagation axis zthereof shown) is emitted through a window 24 in the assembly. Thediode-laser has a base 26 which includes a square notch 28 indicatingthe fast-axis orientation of the diode-laser beam, and a triangularnotch 28 indicating the slow-axis orientation of the diode-laser beam.Electrical connections 32 are provided for connecting current to thediode-laser within the assembly “can”. The diode-laser beam as is knownin the art will be astigmatic with a higher divergence in the fast axis,for example, about 30°, more or less, FWHM, than in the slow-axis, forexample about 11° more or less FWHM. As noted above these values aresubject to variation within any one diode-laser model and amongdiode-laser models. The diode-laser beam is transmitted through apositive lens 34 which has one or more of the well-known functionsdiscussed above in such an arrangement.

A Powell lens 40 intercepts the beam from the diode-laser and thepositive lens. The Powell lens, here has an acylindrical entrancesurface 42 having a vertex 44. The Powell lens here has a planar exitsurface 46. The Powell lens has transverse Cartesian y- and x-axes. They-axis in this instance is perpendicular to vertex 44 of acylindricalsurface 42. This y-axis is the axis in which the acylindrical surfacehas optical power, and is the axis in which the diode-laser beam isspread to form the length of a line of light being projected. Theacylindrical surface has zero optical power in the x-axis and theprojected beam behavior in this axis is essentially determined bypositive lens 34.

Powell lens 40 is configured for a beam having a predetermined y-axisheight at the vertex of acylindrical surface 42 of the Powell lens. Ifit is determined that the actual beam has a height different than theheight for which the Powell lens is configured, the diode-laser isrotated with respect to the Powell lens as indicated by arrows A. Thiscorrespondingly rotates the beam axes with respect to the Powell lens asindicated by arrows A′. The degree of rotation is adjusted such that thebeam has an effective height on the Powell lens vertex which willprovide a desired beam uniformity along a projected line.

Apparatus 20 further includes stray light filter 36 having an aperture38 therein, here, having dimensions just sufficient to pass withoutattenuation the largest beam dimensions anticipated at the Powell lens.This limits the amount of stray (scattered and the like) light thatreaches a surface on which the line of light is projected.

FIG. 2A schematically illustrates an example 20A of the apparatus ofFIG. 2 in which exemplary rays from the diode-laser are depicted bydashed lines spread by the Powell lens. In this example, positive lens34 is configured to focus rays in the x-axis of the Powell lens at apredetermined (specified) working distance from the Powell lens. This isthe distance at which the y-axis uniformity is measured when adjustingthe rotation in accordance with the present invention to provide aspecified uniformity. The diode-laser axes, and beam 50 from positivelens 34, are aligned non-orthogonally with respect to the Powell lensaxis. Whatever the degree of the non-orthogonal alignment, the length(L) of the projected line is always aligned with the y-axis of thePowell lens.

One preferred method of measuring uniformity during adjustment of theuniformity by the inventive method is as follows. The laser diode moduleis installed on a line-scanner. The line-scanner consists of a detectoron a linear stage, capable of measuring light intensity along the laserline and tracking Centroid position in the focusing axis (theline-height or thickness axis).

The output of the line-scanner gives a uniformity profile of the typedepicted in FIG. 1. The module is nominally located about 0.5 m from thedetector plane, but can be placed at other specific locations. Thoseskilled in the art to which the present invention pertains may employother uniformity measurement methods without departing from the spiritand scope of the present invention.

As discussed above, in a typical commercial environment, the optimumrotational position of the Powell lens is determined empirically. FIG. 3and FIG. 4 are provided to explain the uniformity adjustment method inmore analytical terms. FIG. 3 schematically illustrates an ellipticalbeam having a fast axis dimension 2a and a slow axis dimension 2b. Here,the beam has the fast- and slow-axes thereof aligned with the y-axis ofthe Powell lens. In this example, it is assumed that the Powell lens isconfigured for a beam height (H_(P)) in the y-axis of 1.8 mm. The actualbeam height (2*a=H_(B)) in this aligned condition is 2.38 mm.

FIG. 4 schematically illustrates the beam rotated by 30° with respect tothe Powell lens according to the inventive method. This provides thatthe effective height H_(BE) of the beam on the Powell lens is equal tothe configured value of 1.8 mm.

The Effective height as a function of rotation angle (θ) of the fastaxis from the y-axis of the Powell lens is given by an equation:H _(BE)=(a ²*Cos(θ)² +b ²*Sin(θ)²)^(0.5)  (1)where a and b are as defined above.

FIG. 5 is a graph of calculated effective beam height for a case where2a=2.46 mm and 2b=1.21 mm. The solid curve is for the above discussedcase where the beam is initially aligned with the fast axis thereof in(at 0 degrees) the y-axis of the Powell lens. The dashed curve in FIG. 5represents a case where the slow-axis is initially aligned with they-axis of the Powell lens. A reason for choosing this latter case isdiscussed further hereinbelow.

Summarizing the inventive uniformity adjustment method here, the Powelllens is configured for a desired Fan angle and a particular beam heightbetween the maximum and minimum beam heights anticipated from a varietyof diode-lasers. The beam height at the Powell lens location is thencalculated or preferably measured, by any well-known means. A nominalrotation angle can then be calculated or estimated, as discussed above,for providing a specified distribution. An initial uniformity scan ofthe projected line is performed (as discussed above) with thelaser-diode nominal rotation angle. Starting at the calculated orestimated rotation, fine adjustment of the rotation can be performedwhile measuring the distribution to further improve the distribution, ifnecessary. In a preferred practical configuration discussed below thediode-laser can be rotated with respect to the Powell lens, and therotation angle can fixed once a satisfactory distribution is measured.

It should be noted here that the inventive method will not compensatefor a poorly configured or polished Powell lens. Accordingly, the Powelllens should be configured for proper performance for the beamcharacteristics chosen as nominal. Custom Powell lenses are commerciallyavailable, for example from Laserline Optics Canada Inc., of Kanata,Ontario, Canada. A desired specification for the acylindrical surfacecan be calculated for a particular optical glass and combination ofdiode-laser and positive lens by using commercially available opticaldesign software such as ZEMAX available from the Radiant ZemaxCorporation of Redmond, Wash.

It should also be noted that with the rotation of the diode-laser beamaxes from the Powell lens axes, there will be some corresponding changesin the length and width of the projected line. These are typically lessimportant in an application than ensuring the best available uniformityof distribution along the line. If a wide range of beam sizes isanticipated it might be advisable to stock two different Powell lensconfigurations for each of the above-discussed alignment cases(fast-axis or slow-slow axis aligned with the y-axis of the Powell lens)in order to limit the extent of the rotation, for example to plus orminus 30°, that is necessary for any one diode.

One reason for an alignment of the slow-axis of a beam at or close tothe y-axis of the Powell lens (dashed curve of FIG. 5), is thatfast-axis diode-laser rays can be used to focus for the line width orheight. The diode-laser beam has a larger divergence in the fast-axisthan the slow-axis and this alignment would provide for a narrowerfocused or “tighter” line. In the examples depicted above where thePowell lens more closely matches the fast-axis beam height, theslow-axis diode-laser rays are focused and provide a larger depth offocus than would be the case if the fast-axis rays were focused.

FIG. 6 schematically illustrates an example 60 of a diode-laser lineprojector configured for accommodating the components of FIG. 2 andimplementing the beam uniformity adjustment method in accordance withthe present invention. Projector 60 comprises three basic units. A basicunit 62 houses the Powell lens. A line of light 66 is projected from anaperture 64 in unit 62. Unit 62 is fixedly attached to a focusing unit68 which includes a focusing sleeve 70 rotatable with respect to thefocusing unit for providing linear (z-axis) translation of the positivelens of FIG. 2. A diode-laser unit 72 has a positive-lens subassembly(not shown in FIG. 6) attached thereto and enclosed within focusing unit68. Unit 72 is rotatable with respect to unit 62 for implementing theinventive beam-uniformity adjustment method. Once a preferred rotationis determined, that rotation can be fixed by tightening set screws 74which bear on the positive-lens subassembly.

FIG. 7 is a longitudinal cross section view of the unit of FIG. 6depicting the components which allow the inventive uniformity adjustmentmethod to be implemented. The diode-laser, positive lens, and Powelllens are designated by the same reference numerals as in FIG. 2.

In unit 72, covered by a housing 73, diode-laser 22 is attached by base26 thereof to a mounting flange 80. Flange 80 has a PC board (withdriver components of the diode-laser) attached thereto. The PC board isattached to housing 73 by means not shown. Flange 80 is fixedly attachedto a focus subassembly housing 82, which extends to the front of focusunit 68 as illustrated. If set screws 74 (only one visible in FIG. 7)are not tightened, unit 72 and diode-laser 80 attached (indirectly)thereto can be rotated relative the Powell lens in unit 62. When asatisfactory orientation of the diode-laser with respect to the Powelllens has been found, screws 74 can be tightened against subassemblyhousing 82, as illustrated, to fix the diode-laser orientation withrespect to the Powell lens.

Positive lens 34 is fixedly held at one end of a cylindrical lens-holder86, which has a sliding fit in subassembly housing 82. Links 88, at oneend thereof, engage lens holder 86 via elongated slots 90 in subassemblyhousing 82. Links 88 at an opposite end thereof engage a spiral channel92 in focusing sleeve 70.

Turning sleeve 70 translates lens-holder 86 and lens 34 therein asindicated by arrows F. A spring 94 is compressed between lens-holder 86and retaining-flange 96 screwed into subassembly housing 82. Thisprevents movement of the lens by external forces. An O-ring 98 istrapped in a channel 100 in subassembly housing 82 by focusing sleeve70. This provides sufficient friction to retain a rotation of thefocusing sleeve after a focus adjustment.

A description of the effectiveness of the inventive beam uniformityadjustment method is set forth below with reference to FIG. 8A, FIG. 8B,and FIG. 8C, which are graphs depicting measured relative uniformity asa function of position along a projected line from a projector similarto the projector of FIGS. 6 and 7. In this projector, the laser-diode isa diode-laser emitting radiation at a wavelength of 515 nm. The positivelens is a model PL-250 from Osram GmbH of Munich Germany. The Powelllens is configured to project a beam having a height (y-axis height) of1.3 mm into a fan angle of about 30 degrees. The fast-axis and slow-axisbeam-dimensions at the Powell lens are 2.7 mm and 1.1 mm respectivelymeasured at the 1/e² level. Power was measured by the method discussedabove at a distance of 0.5 meters (500 mm) from the exit face of thePowell lens.

In the measurement of FIG. 8A the slow-axis of the beam is aligned withthe Powell lens, i.e., aligned with y-axis of the Powell lens asdescribed above. This makes the beam height at the Powell lens less thanthe design height of 1.3 mm providing a rounded distribution similar tothat of FIG. 1. Given a uniformity specification of greater than 80%along a line this would be applicable to a line having a length of about190 mm (defined between vertical dash-dot lines at 145 mm and 335 mm).

In the measurement of FIG. 8B the fast-axis of the beam is aligned withthe Powell lens. This makes the beam height at the Powell lens muchgreater than the design height of 1.3 mm providing a “pitchfork”distribution similar to that of FIG. 1. Here of course the powerdistribution is completely unacceptable.

FIG. 8C is a graph schematically illustrating measured relative power asa function of line length when the diode-laser beam of FIG. 8A has theslow-axis thereof aligned at angle of 30° to the Powell lens i.e.,rotated at 30 from with y-axis of the Powell lens. Here the relativepower is above 80% over a length of about 260 mm, with an RMS variationalong the length less than in the case of the 190 mm length of FIG. 8A.

In a preferred method of implementing the subject invention, the personassembling the device will determine the nominal parameters of the diodelaser being used. The nominal parameters are typically provided by themanufacturer of the diode laser. Based on this information, theassembler would select a Powell lens having the parameters best matchedto the diode laser. During set up, the acylindrical axis of the lens canbe aligned with one of either the fast or slow axes of the diode laser.Thereafter, the azimuthal angle of the Powell lens can be adjusted withrespect to the axes of the diode laser while intensity measurements aremade as described above. When the optimal performance is achieved, theset screws 74 are tightened, locking the elements of the unit in place.This procedure results in a projector with relatively uniform intensityacross the line of light at relatively low cost since the tolerances forthe Powell lens can be relaxed.

The present invention is described above in terms of a preferred andother embodiments. The invention, however, is not limited to theembodiments described and depicted. Rather the invention is limited onlyby the claims appended hereto.

What is claimed is:
 1. A diode laser line projector device comprising:an elongated housing having first and second sections, said sectionsbeing rotatable with respect to each other along the longitudinal axisof the housing; a diode laser mounted within the first section of thehousing, said diode laser generating a beam of radiation havingorthogonal fast and slow axes mutually perpendicular to the longitudinalaxis of the housing; a first lens mounted within the second section ofthe housing, said first lens having an acylindrical surface including avertex lying along a first axis perpendicular to the longitudinal axisthe housing; and means for selectively locking the first and secondsections of the housing together to prevent rotation after the vertex ofthe acylindrical surface of the first lens has been preferentiallyaligned with respect to the axes of the diode laser.
 2. A device asrecited in claim 1 wherein said locking means is defined by set screwsextending through a wall of the housing.
 3. A device as recited in claim1 wherein said first lens is a Powell lens.
 4. A device as recited inclaim 1 further including a second lens mounted in the housing andpositioned between the diode laser and the first lens.
 5. A device asrecited in claim 4 wherein said housing further includes a means foradjusting the spacing between the second lens and the diode laser.
 6. Adiode laser line projector device comprising: an elongated housinghaving a longitudinal axis; a diode laser mounted within the housing,said diode laser generating a beam of radiation having orthogonal fastand slow axes mutually perpendicular to the longitudinal axis of thehousing; and a first lens mounted within the housing and spaced from thediode laser for projecting the beam of radiation generated therefromalong a line, said first lens having an acylindrical surface including avertex lying along a first axis perpendicular to the longitudinal axisthe housing and aligned between the fast and slow axes of the diodelaser at a position so that the uniformity of the intensity distributionof light along the projected line is greater than 80 percent.
 7. Adevice as recited in claim 6 wherein said first lens is a Powell lens.8. A device as recited in claim 6 further including a second lensmounted in the housing and positioned between the diode laser and thefirst lens.
 9. A device as recited in claim 8 wherein said housingfurther includes a means for adjusting the spacing between the secondlens and the diode laser.